Membrane pumps where the body consists of eleastic material and forms part of the space to be compressed, are advantageously used in dispensers for viscous products, such as toothpaste, lotions, or the like. Such a dispenser may consist of a cylindrical container with covering face and a piston which sealingly fits against the inside wall of the container at its circumference and pressed the viscous container filling against the covering face during forward movement caused by the atmospheric pressure. The membrane pump may consist of an inlet valve in the covering face, an outlet valve, and a pump chamber remaining between the valves which is to be elastically compressed and which is closed except for the valves.
Dispenser containers for viscous fluids are well known. For example, a dispenser for viscous cosmetics such as toothpaste or lotion is described in Kolaczinski et al. U.S. Pat. No. 4,154,371. The Kolaczinski et al. container comprises a piston forming the bottom of the container and being displaceable in the container; a compressible container part, lying in front of the piston; a mouthpiece, for withdrawal of the contents, of flexible and elastic material; and a delivery valve opening on excess pressure in the container. The piston is secured by a blocking pawl against any displacement causing enlargement of the interior space of the container. The delivery valve comprises a flap having an ares of 0.8 cm.sup.2 or less which is mounted in the mouthpiece by resilient hinge means.
A similarly useful dispenser is also described in Spatz, U.S. Pat. No. 3,361,305. In the Spatz dispenser, several inlet flap valves are arranged in the top surface of the container between the storage chamber of the container and the pump chamber, which flap valves are arranged substantially symmetrically to the center of the top surface. When the diaphragm pump of the dispenser is operated, a flexible, elastic surface is moved downward, perpendicularly to the top surface containing the flap valves in such a way that the product contained in the pump chamber is ejected or dipensed through the delivery or outlet valve of the despenser. When the flexible, elastic surface returns to its normal, unstressed position, the delivery valve is closed because of a resulting vacuum and the inlet flap valves arranged in the top surface open in such a way that new material is drawn into the pump chamber from the interior of the container, that is, from the storage chamber, by the pressure differential, or vacuum, produced in the pump chamber.
In useful dispenser containers, the ciruclar, symmetrical elastic body or portion of a body containing the valve opening of the outlet flap valve may, of course, be produced in different ways, such as, for example, by extruding a material which, after extrusion, an be elastically shaped in the desired degree. However, when products are extruded in quantity, slight deviations from the specified dimensions are inevitable. Thus, an extruded body surrounding the outlet valve opening in circularly symmetrical fashion, for example, like a bell, is likely to lack homogeneity and can therefore not fulfill close tolerances exactly. In therory, plastic parts can be produced by automated injection molded with high precision and also with considerable homogeneity with regard to the internal stresses. For circularly symmetrical bodies, however, this is true only if the injection molding is effected from a point of the axis of symmetry.
Since the body of the above-mentioned flap valve adjacent to the circularly symmetrical valve opening is everywhere farther away from the axis of symmetry than the inner circumference of the valve opening itself, the sprue in injection molding can actually start only in the area of the valve itself. The only part of the flap valve which touches the axis of symmetry is its flap. But injection molding cannot proceed from the flap, as that structural part is itself normally unsymmetrical because it is to be connected with the rest of the valve body on one side only, for instance, through a film hinge. If, therefore, the sprue must be placed at a point along the line of symmetry of the body, this point can logically lie only in the area of the narrowest circumference of the body, that is, in the valve opening. Since in injection molding there always remains a more or less large sprue wart, reliable operation of the flap valve cannot be readily assumed--even if the remaining residual aperture cross-section is sufficiently large-- because the flap will not always seal to the required degree, for example, if the sprue wart is relatively thick. The result has heretofore been that this possible disadvantage could be remedied only by grinding down or complete removal of the sprue in a special production step after the molding.